It is known to arrange a passive sensor in a drilled hole or in another securing opening in the region of a vehicle wheel in a clamping fashion in order to perform rotational speed sensing. Analog signals are transmitted from the passive rotational speed sensor to a control unit, which can additionally determine the respective air gap between the passive sensor and a pole wheel from these analog signals. The passive sensor can be displaced axially in the drilled hole and can therefore, under certain circumstances, assume an unacceptably large distance from the pole wheel, which can, however, be detected at the control unit.
Furthermore, if is known to arrange or to secure an active rotational speed sensor in an immovable fashion in the region of the pole wheel. The active rotational speed sensor therefore cannot be displaced unacceptably. When there is such an active rotational speed sensor, simple monitoring of the air gap is carried out by means of the rotational speed sensor itself. DE 199 11 774 A1 discloses detecting changes in the air gap by means of the rotational speed sensor in order to generate a corresponding status signal in good time before a possible sensor signal loss. However, in this context, only a binary distinction is made at the rotational speed sensor between an acceptable and an unacceptable air gap.
Furthermore, for other purposes, an analog signal, which is a measure of the air gap, is digitized with 3-bit coding and provided to a control unit. According to DE 102 03 483 A1, the word size with which, for example, the magnetic air gap field strength can be transmitted from the rotational speed sensor to the control unit can be 4 bits. The control unit can track the chronological profile of the air gap field strength with increased resolution and, if appropriate, evaluate it further. In particular, this procedure aims, according to DE 102 03 483 A1, to sense dynamic changes in the air gap and use them to determine driving states in a brake control system and/or vehicle dynamics control system. Furthermore, the observation of the current air gap according to DE 102 03 483 A1 can also be used as a measure of the lateral acceleration acting on the wheel or the wheel bearing temperature. Therefore, only changes in the air gap that are caused by the position of the pole wheel are evaluated at the control unit. On the other hand, the correct position of the rotational speed sensor is further monitored in a simple way by means of the speed sensor.